Liquid crystal display apparatuses for image display are widely used in the displays of mobile phones, notebook personal computers (PCs), car navigation systems, mobile PCs, personal digital assistants (PDAs), automatic teller machines (ATMs), etc. Such a liquid crystal display apparatus employs a lightguide plate as a part of a backlight unit that applies illuminating light to a liquid crystal display panel from the back of the display panel to enhance the luminance of the display screen of the display panel.
The backlight unit has a lightguide plate and a light-emitting diode (LED) or other light source disposed along a light-entrance surface of the lightguide plate. The lightguide plate guides light from the light source through it and emits the guided light toward the liquid crystal display panel from the whole of its light-exiting surface that faces the display panel. Recently, there has been an increased demand for reduction in thickness of backlight units in accordance with the reduction in thickness of apparatuses in which backlight units are incorporated. Under these circumstances, film-shaped lightguide plates in particular have been increasingly employed. However, when using a lightguide plate that is shorter in height than the light-exiting surface of the associated light source as in the case of a film-shaped lightguide plate, a part of light from the light source fails to enter the light-entrance surface of the lightguide plate and leaks, resulting in a loss of light entering the lightguide plate, which causes a reduction in luminance at the light-exiting surface of the lightguide plate.
To solve this problem, Japanese Patent Application Publication No. 2000-249837, for example, proposes a lightguide plate having a lightguide plate body and a lightguide block bonded to the lightguide plate body along one side edge of the lightguide plate body and extends toward a light source from the side edge with its wide light-entrance surface facing the light-exiting surface of the light source. This lightguide plate receives light not through the light-entrance surface of the lightguide plate body, which is thin, but through the wide light-entrance surface of the lightguide block and guides the received light into the lightguide plate body through the lightguide block.
Japanese Patent Application Publication No. 2002-42502 proposes a film-shaped lightguide plate locally increased in thickness at a part along its side edge facing the associated light source to make it possible to receive light from the light source without leakage.
The above-described conventional techniques, however, still have the following problems to be solved.
With the technique disclosed in Japanese Patent Application Publication No. 2000-249837, the lightguide plate receives light from the light source through the lightguide block and guides the light by refraction into the lightguide plate body. Accordingly, the light-guiding process is attended with a large loss of light. In addition, the structure interferes with the reduction in thickness of the lightguide plate for the reason that a lightguide block having a light-entrance surface with a height corresponding to the light source is stacked on the lightguide plate body.
According to the technique disclosed in Japanese Patent Application Publication No. 2002-42502, the film-shaped lightguide plate is increased in thickness only at its side edge part facing the light source. Therefore, the lightguide plate is difficult to produce by injection molding and cannot be manufactured by roll forming.